This invention relates to telecommunications systems, and more particularly to systems and methods for supporting handover of wireless calls between wireless telecommunications systems or components thereof which support differing call models, including circuit and packet call models.
An important feature in many wireless telecommunications systems is mobility, whereby a user involved in a call may move from a first location supported by a first set of wireless infrastructure equipment, to a second location supported by a second set of wireless infrastructure equipment, without significantly disrupting the call. Many early wireless telecommunications systems were developed to provide mobile telephone service. Early mobile telephone systems typically employed a single radio base station positioned to cover a large geographic area, albeit with limited capacity. A mobile user could travel widely within the covered area and expect to maintain a call provided that the user did not move to a location where the radio-frequency path to the base station became unusable. When cellular mobile telephone systems, comprising a large number of radio base stations each serving much smaller adjacent areas or xe2x80x9ccellsxe2x80x9d, were developed, it was essential to allow users involved in calls to move from cell to cell throughout the system""s coverage area without disrupting the call.
The function and implementing processes of causing a stable call currently being served by a first radio base station (or another similar element of a wireless system providing an over-the-air interface) to be served by a second radio base station are referred to as a xe2x80x9chandoffxe2x80x9d or xe2x80x9chandoverxe2x80x9d. Handovers were initially provided among cells of a single system and of like technology. However, standard protocols have been developed to allow handovers among cells of different systems, and to allow handovers among cells and/or systems of differing technology. For example, standard protocols allow calls to be maintained as users cross boundaries from one wireless system to another system, perhaps operated by a different entity and using a different type or brand of infrastructure equipment. For example, protocols of this type include the standardized intersystem operations protocol known as ANSI-TIA/EIA 41-D: Cellular Radiotelecommunications Intersystem Operations, a publication of the American National Standards Institute, and the GSM 09.02 Mobile Application Part (MAP) protocol, a publication of the European Telecommunications Standards Institute (ETSI). Moreover, standard protocols have also been developed to permit handovers among systems/cells of differing (but cooperative) air interface technologies using the same call model. For example, some subscriber handsets and system infrastructure equipment may execute a handover of a call from a cell employing digital transmission technology, such as CDMA or TDMA, to a cell employing analog transmission technology, such as AMPS. The capability of performing handovers between GSM and UMTS systems has also been described. Although the need for mobility historically may have motivated the use of handovers, handovers can provide important functionality even in applications which do not require mobility by allowing load balancing and improving reliability.
Existing wireless telecommunications systems which provide handovers have employed a circuit call model. The term xe2x80x9ccallxe2x80x9d is used herein to refer to a session of information transfer between a set of terminals via a telecommunications system or network, and is intended to include, but not be limited to traditional circuit voice calls, packet voice calls, circuit data calls, connectionless calls, or packet data calls, and multimedia variants thereof. The term xe2x80x9ccircuitxe2x80x9d as applied to a call refers to a mode of information transfer which occurs between defined endpoints over reserved network resources, and in which units of data are not individually addressed. Once a path or route is established for a circuit call, no further routing or addressing is required. It is recognized that some components carrying a circuit call may be implemented using packet- or cell-based technologies. The term xe2x80x9cpacketxe2x80x9d as applied to a call refers to a mode of information transfer in which a stream of information is divided into packets or units, and in which each packet or unit is individually addressed. A packet call does not necessarily reserve network resources. The term xe2x80x9ccall modelxe2x80x9d refers to the procedures, states, and state transitions required to set up, maintain, modify, and end a call. A circuit call model is a call model used to establish and control circuit calls. Examples of known circuit call models include: ITU-T Signaling System No. 7, ANSI-41, ANSI-136, ANSI-95, and GSM 04.08. A packet call model is a call model used to establish and control packet calls. Examples of known packet call models include IETF RFC-2543 (Session Initiation Protocol (SIP)) and ITU Specification H.323.
New telecommunications systems, including wireless systems, have been proposed or are being developed which employ a packet call model. Packet call models imply that during a call, certain resources and facilities may be allocated on an as-needed basis to carry the call""s bearer traffic, and that the particular resources and facilities used may vary with each packet. Packet systems may support an end-to-end packet callxe2x80x94that is, a call where each terminal is adapted for packet communications, and the call is carried over a packet network. However, a large fraction of the world""s telecommunications infrastructure employs circuit technology, and therefore, many packet systems are being designed to interwork calls with existing circuit networks, at least at certain well-defined interfaces. Thus, a call could originate at a packet terminal, but be terminated at a circuit terminal, or vice versa. Systems for interworking calls in conventional land-side packet and circuit networks are known in the art, and such a system is sold by Lucent Technologies, Murray Hill, N.J., under the designation PACKETSTAR Voice Gateway.
New packet wireless systems are likely to be constructed in phases, and it is likely that such systems will initially be deployed to overlay existing circuit wireless systems, in which system operators have made extremely large investments. Accordingly, it will be desirable to provide handovers between packet and circuit system for suitably equipped subscriber handsets and other terminals. Such handovers, advantageously, would allow subscribers to be served by the new packet system in locations where it is available, and to be served by the existing circuit system in locations where the packet system is not available or temporarily lacks capacity. In addition to providing for mobility, handovers between these systems would also allow load balancing and improve reliability.
However, existing circuit systems have employed network topology and handover processes which are uniquely suited to the circuit call model. In particular, commercially deployed circuit systems employ an anchor Mobile Switching Center, or xe2x80x9canchor MSCxe2x80x9d, to control a call throughout its duration. The anchor MSC is generally the first MSC to have substantive control over a call. During a call, even though the user may move into the service area of another system, which generally would be controlled by a different MSC, certain other features are controlled by the anchor MSC, and the call""s bearer traffic is routed through the anchor MSC. The current serving MSC controls handoffs.
The topology of packet wireless systems, according to proposed standards, differs significantly from that of existing circuit wireless systems. In particular, in proposed packet wireless systems, the system elements providing control functions may be different from the system elements providing switching, transmission, and vocoding functions. The packet wireless systems do not employ an anchor MSC component. Furthermore, packet wireless networks employ both circuit and packet call models to interface to other networks, whereas circuit wireless networks employ only circuit call models. These significant differences, and others, make it impossible to directly apply conventional handoff processes developed for circuit wireless networks to new packet networks.
Moreover, existing circuit networks represent huge investments for their operators, but employ technology for which major upgrades may be unavailable without complete replacement or significant additional investment. Accordingly, in order to feasibly support handover with existing circuit wireless systems, any handover processes and functionality developed for packet systems must minimize required changes or upgrades to existing circuit wireless systems. Thus, handoff procedures developed for homogeneous packet networks are not sufficient for use in packet systems that must support handovers with circuit systems.
It is therefore an object of the present invention to provide systems and/or methods for performing handovers in wireless systems which avoid the aforementioned disadvantages of the prior art.
In a preferred embodiment constructed according to an aspect of the present invention, a wireless network comprises a packet wireless system and a circuit wireless system which are arranged to interoperate, including supporting handover, using a defined interoperation protocol. The circuit wireless system may be of conventional design and use any appropriate wireless technology or standards. The circuit wireless system includes at least one base station and at least one mobile switching center (or equivalent elements).
The packet wireless system may be constructed in a manner generally similar to known packet wireless networks, but with certain components added, and other components modified, to provide inter-call-model handover functions according to an aspect of the present invention. For example, the packet wireless system may employ the basic structure and functionality of a general packet radio service (GPRS) supplemented by the IP Multimedia subsystem (IM) as described by the Third-Generation Partnership Project (3GPP), with appropriate modifications. Alternative packet wireless system technologies could also be used. If a packet wireless system employing the GPRS-like architecture is used, the packet system includes an interconnected set of at least one of each of the following GPRS elements: a base station; a radio network controller; a serving GPRS support node (SGSN); and a gateway GPRS support node (GGSN). These elements generally perform as they would in a GPRS system, with some modifications to implement the interoperable handover functions of the present invention. The packet system also includes an interconnected set of at least one of each of the following elements from the 3GPP IM subsystem: a call state control function (CSCF), a media gateway (MG), and a media gateway control function/transport signaling gateway (MGCF/T-SGW), which are interconnected with the other elements. The CSCF is a network element which implements the network functions of the packet call model. The MG translates bearer content between the encoding and transmission formats used in the packet network and those used in circuit networks. For example, for voice calls, the MG may perform a vocoding function to translate between compressed formats used in packet networks and PCM formats used in circuit networks. The MG may also translate among formats used by disparate packet networks. The MGCF/T-SGW controls the MG and provides the control interface to external networks. The MGCF/T-SGW is also used to emulate certain functions of an anchor MSC when intersystem operations with a circuit wireless network are required.
According to aspects of the present invention, four possible handover situations are supported:
A stable call which is terminated on a circuit land-side network and which initially uses the packet wireless system may be handed over to the circuit wireless system. Because existing circuit systems require an anchor MSC to maintain control of a call throughout its duration, MGCF/T-SGW, MG, and CSCF cooperate to emulate the functions of an anchor MSC and appear to the circuit wireless system as simply another circuit wireless system. After the handover, bearer traffic from the circuit wireless system is routed to the circuit land-side network through the MG. As used herein, the term xe2x80x9cland-side networkxe2x80x9d is intended to include any other network which provides interfaces equivalent to a land-side network, including but not limited to other wireless networks and transit networks.
A stable call which is terminated on a circuit land-side network and which initially uses the circuit wireless system may be handed over to the packet wireless system. Because existing circuit systems require an anchor MSC to maintain control of a call throughout its duration, the MSC of the circuit system maintains control of the call. MGCF/T-SGW, MG, and CSCF cooperate to emulate the functions of a circuit MSC for intersystem handovers and appear to the circuit wireless system as simply another circuit wireless system. After handover, bearer traffic exchanged between the packet wireless network and the circuit land side network is routed through the MG to the circuit wireless system.
A stable call which is terminated on a packet land-side network and which initially uses the packet wireless system may be handed over to the circuit wireless system. Because existing circuit systems require an anchor MSC to maintain control of a call throughout its duration, MGCF/T-SGW, MG, and CSCF cooperate to emulate the functions of an anchor MSC and appear to the circuit wireless system as simply another circuit wireless system. Prior to handover, the MG may or may not be an element of the bearer path. After the handover, bearer traffic from the circuit wireless system is routed to the packet land-side network through the MG and GGSN.
A stable call which is terminated on a packet land-side network and which initially uses the circuit wireless system may be handed over to the packet wireless system. Such a call must transit through an interworking function, which masks the existence of the packet network to the circuit network. Therefore, this case reduces to the instance of a call which is terminated on a circuit land-side network and which initially uses the circuit wireless system and is handed over to the packet wireless system.
These four handover situations describe all possible handover combinations between packet and circuit networks. The system and methods disclosed herein advantageously allow intersystem handoffs between packet wireless systems and conventional wireless circuit systems. The handoff functionality is provided in the packet system, minimizing or avoiding entirely the need to modify or upgrade existing circuit systems.